The Mackintosh design is a listed building and is a tourist attraction in its own right.

Construction work started on the building in 1897 after Mackintosh won a design competition while working for Glasgow architects Honeyman and Keppie.

It took around 10 years to complete due to funding issues and several alterations were made by Mackintosh during the process.

More than a century later it remains a working building serving hundreds of students.

It also attracts about 20,000 visitors each year, who are taken on tours of the building, according to the School of Art website.

Other popular Mackintosh designs in his home town of Glasgow include the Lighthouse, the Willow Tearooms and House For An Art Lover in Bellahouston Park.

Scottish Culture Secretary Fiona Hyslop said on Twitter: "Serious concern about fire at Glasgow School of Art."

Shadow Scottish secretary Margaret Curran used the social networking site to say she had "just heard about terrible fire at Glasgow School of Art", adding: "Hope people are safe."

Scottish Tory leader Ruth Davidson, who is an MSP for the Glasgow region, said the blaze was "dreadful news", describing the art school building as "one of Glasgow's finest".

Austin Yuill, who works as a chef at the art school, told the BBC: "I've been moved two streets away from the Mackintosh building but before we left the place was completely ablaze all down the west side of the building.

"I'm told it started in the basement and it's worked its way all the way up through the five floors.

"As far as I know it started from a spark which has gone on to foam, expanding foam."

Asked how busy the art school was, he said: "It would be very busy because we're working up to the end-of-year assessments, so all the students were installing their work today all over the Mackintosh building. There are a lot of very upset students here."

He added: "Quite apart from it being voted the best building of the last 175 years, it is a major tourist draw and has an incredible reputation as an art school. This is really terrible."

Some students were in tears outside the building.

Hugh Thornhill, a second year student, said: "I was helping one of the fourth years set up their exhibit and suddenly the alarm went off. We didn't think it was anything but we had to go out and then we saw smoke coming out and realised that it was really bad.

"It got to the point where flames were coming out of the top floor.

"All that effort is gone, everyone's work on that side of the building is ruined. Even if it didn't catch fire it will be damaged extensively.

"The degree show next month is pretty much a bust now, it's sad."

Staff are arranging for locksmiths and services for students who left personal belongings and keys in the building.

Second year student Clare Reilly said: "It's not about that (keys and personal belongings), four years of work is gone for all those students, the deadline was at 5pm today.

"Even the archive and library in there will be gone.

"There's so much work and history in the building and it's gone. I've got two years to go but we might not be allowed back in."